Medical spatial tracking is needed, for example, in order to perform three dimensional ultrasound imaging using existing medical ultrasound transducers designed for 2D imaging and, in the near future, for spatially tracking the motion of virtual-reality guided surgical tools.
As those skilled in the art of three-dimensional motion tracking are aware, one may utilize any convenient coordinate system to perform spatial (position and orientation, i.e. "P/O") tracking. Commonly used coordinate systems are Cartesian, spherical and cylindrical coordinate systems, for example.
Ultrasound, or sonography, continues to be one of the most cost-effective medical imaging modalities. Some of the reasons for this include: 1) the exam cost is comparatively inexpensive, typically a few hundred dollars, and is quickly executed, 2) the information is available in real-time, 3) the image quality and image frame rate continue to improve, 4) new modalities are appearing such as power or Color Doppler-Energy (CDE) which allow for the viewing of previously unseen phenomenon such as very low flow, 5) the array of available transducers for external, endocavity and intraoperative applications is growing, and 6) the operator interface, as supported by software and improved displays, remains relatively simple despite the vastly improved system capabilities.
Virtually all medical ultrasound imaging is two dimensional in nature wherein only the real-time or recorded two dimensional scanplane slices are viewed. It is now generally recognized that without three-dimensional ultrasound, important information and spatial relationships in the pathology may be missed in some organs such as in the heart. Thus there is growing interest in 3D ultrasound.
Other imaging modalities such as MRI and CAT scan, have long presented three-dimensional views of the body interior to the practitioner. In many cases these three-dimensional views are actually sets of stepped two-dimensional image planes or slices each situated closely adjacent to each other. The stepping is typically orthogonal to the plane of the individual two-dimensional images. The sets of stepped planes may be presented in a pseudo-3D or 3D display wherein the observer appreciates the depth-dimension information as well as the in-plane information. By timing the gathering of these image planes with the heartbeat one may even present a dynamic time-varying 3D view of the body. This is referred to as 4D imaging wherein the fourth dimension is time.
It is increasingly apparent that analogous 3D or 4D ultrasound imagery would give the medical practitioner the best possible perspective for diagnosis (and therapies) and that, if properly implemented, would allow for a wider range of sonographers having different skill levels to all get "ideal" images. In particular, if a sonographer could take a standardized volumetric (3D) image of a patient and then a doctor or expert-software program could manipulate that volumetric image data to obtain the particular desired image slice(s) of interest, or make a particular quantitative diagnostic conclusion, then the transducer probe mechanical-handling and anatomy interpretation skills required to intelligently use today's 2D ultrasound imaging hardware could be relaxed.
A significant difference between medical ultrasound equipment and MRI or CAT scan equipment is that for ultrasound, the determination of which scanline or image slice is being sampled at a particular time depends on how the sonographer is manually holding and/or moving the transducer probe relative to the patient. In virtually all current ultrasound exams the transducer probe is held in a free-hand manner so that successively sampled scanlines or image planes may easily be angulated and/or rotated in space relative to each other due to purposeful and/or unintentional hand motion of the operator. In order to provide 3D or 4D ultrasound imagery from which image features may be quantified, one needs to at least accurately know how the multiple scanlines or image planes making up a volumetric sample are spatially and temporally related to each other. One may further need to know the spatial relationship of the set of scanlines or images to certain body markers or reference points on or in the patient. Without accurate spatial address information tagged to the individual scanlines or image slices one cannot hope to construct an undistorted volumetric 3D representation of the patients tissues from multiple such scanlines or image planes. In CAT scan and MRI, for example, all scanning and scanning motions are reproducibly machine-controlled and this problem does not arise.
A variety of mechanisms have been disclosed which attempt to do with ultrasound what has already been done with MRI and CAT scan in terms of mounting the single image-plane transducer probe on a compact rotatable or slideable, i.e., translatable track, sled or helix which achieves the required translational or rotational volumetric sweeping of the 2D ultrasound image plane to get a 3D volumetric database. Although for some applications this may be workable, for most applications wherein scanning over an extended curvilinear and compliant tissue surface is required, it is not. U.S. Pat. No. 5,159,931 discloses one situation where this probably is an acceptable solution for imaging the heart through the chest wall wherein a transducer probe is utilized which is cylindrical in shape and the transducer probe is rotated, not translated, about its central axis as it looks into the body. A cone-shaped 3D volume is thus swept out via stepped angular incrementing of the 2D array around the rotation axis passing through it. The rotation axis extends into the patient's body and represents the long gripped axis of the transducer probe case. Ideally, but not necessarily, the rotating transducer probe is isolated from the skin surface by a stationary sealed acoustic window.
However, for the more general 3D applications wherein it would be desirable to move the transducer probe in a freehand, unaided manner, or at least in a minimally encumbered manner various types of sensors have been used. Some devices use accelerometers. See U.S. Pat. No. 5,353,354. Others use magnetic sensors. See U.S. Pat. No. 5,505,204; Leotta D., et al., "Three-dimensional ultrasound Imaging Using Multiple Magnetic Tracking Systems and Miniature Magnetic Sensors," proceedings of the 1995 IEEE Ultrasonics Symposium, pp. 1415-1418 (November 1995); and Detmer, P. et al., "3D Ultrasonic Image Feature Localization Based On Magnetic Scanhead Tracking: In Vitro Calibration and Validation," Ultrasound in Medicine and Biology, Vol. 20, No. 9, pp. 923-936 (1994). Still others use gyroscopes. See Shinozuka, N. et al. "Transvaginal Sonographic and Orientation Detection System Using Ceramic Gyroscopes," Ultrasound in Medicine, Vol. 15, pp. 107-113 (1996). While this can preserve some or all of the desired freedom of motion of the ultrasound transducer probe, there remain inadequacies in each of these solutions.
In particular, there is a lack of sufficiently accurate translation and angular motion-discrimination which is important for deep ultrasound imaging and for narrow-elevation, i.e., thin slice imaging in 3D. Magnetic sensors are significantly inferior in angle measurement. So are accelerometers by themselves. Gyroscopes by themselves cannot detect translations.
In addition, spatial reporting errors are caused by passive metallic or magnetic objects in the motion space when using magnetic solutions such as that disclosed in U.S. Pat. No. 5,465,724. Also, failure or degradation of a magnetic tracking system's performance may be caused by active electromagnetic interference as from an electrocautery knife, an RF ablation tool or the imaging transducer itself or from passive interference as from an associated metallic biopsy needle device.
Existing transducers incorporate in their construction undesirable magnetic materials causing "passive" interference with such magnetic position sensing systems. They also incorporate metallic, electrically conductive components arranged in a way that invite errors in magnetic tracking systems due to induced eddy currents and their companion "active" interference. Existing transducers also lack the necessary shielding so as to allow an on-board magnetic sensor to operate without significant interference from the functioning transducer itself These issues have been addressed in U.S. Pat. No. 5,465,724, Sliwa, et al.
It is thus desirable to provide a position and orientation system wherein the measurement of orientation is insensitive to electromagnetic noise and which is more accurate than known systems already described. In addition, it is desirable to provide a position and orientation system that is able to directly measure absolute orientation instead of deducing relative orientation and to measure absolute orientation with a high degree of accuracy.
Further, it is desirable to provide a position and orientation system that does not incorporate into its construction undesirable magnetic materials which cause passive interference with the position and orientation system. It is desirable to avoid inducing errors in magnetic tracking systems due to induced eddy currents in metallic, electrically conductive components and the resulting active interference. It is desirable to provide a position and orientation system having features of redundancy and, in some cases, self-optimization, self-calibration and self-testing. It is also desirable to provide a position and orientation system which can correct for electromagnetic field disturbances coming from outside the transducer probe.
It is also desirable to provide a position and orientation system that can calculate the relative position and orientation of two devices. Further, it is desirable to provide a position and orientation system that determines the relative position and orientation of points within a medical device, such as a needle, when the device cannot be made rigid or when it is impractical or undesirable to make to device rigid.